Default Kaiser II mail woes

I have a Kaiser II (a.k.a. AT&T TILT) with an my companies email setup on it--mail protocol is IMAP. I have the phone set to only show mail from the past 7 days--previously 30 days. My problem is even after changing the the number of days from 30 to 7 my phone's memory is filling up. It seems like that while the phone only shows 7 days worth of email it stores everything. This is starting to get really annoying. Please help.